1) Early philosophers proposed the idea of atoms as the smallest indivisible units of matter. Rutherford's gold foil experiment in the early 1900s showed that atoms have a small, dense nucleus at their center.
2) Bohr's model improved on Rutherford's model by proposing that electrons orbit the nucleus in fixed energy levels. This explained the emission spectra of hydrogen. Later models incorporated the dual wave-particle nature of electrons and the Heisenberg uncertainty principle.
3) Modern atomic theory describes electrons as existing in electron clouds around the nucleus, with their locations defined by quantum numbers like principal, azimuthal, magnetic, and spin quantum numbers. This quantum mechanical model best explains the structure and behavior of atoms